This invention relates to coil and trip mechanisms for interrupting electrical circuits, and particularly to those for use in assemblies or installations where limitations of space and temperature rise are a factor. Accordingly, the trip coil must be physically small and the temperature rise must be kept low. Such a coil must have a relatively high resistance to lower the heat potential to an acceptable level. However, when the resistance of the coil is increased sufficiently to control temperature rise within acceptable limits, the physically small coil can no longer produce enough electromagnetic force when energized to trip the interrupting mechanism.
Heretofore, coils small enough to meet the space limitation requirements had to have relatively lower resistance in order to generate enough electromagnetic force to trip the interrupting mechanism. If full energizing voltage is applied constantly to such coils, the temperature rise will become great enough to burn out such coils within a relatively short period of time.
It is desirable to use constant rated coils in such limited space applications which do not burn out under conditions whereby energizing voltage is applied constantly to the coil. The present invention accomplishes the desired result by providing a physically small coil plus two oppositely biased armatures with the latched position bias dominant, or sufficiently stronger to bias the armatures away from the coil face and prevent tripping until the coil is energized. One of the armatures is of the clapper type mounted with respect to the coil for movement between a latched position and a tripped position, and having a subordinate bias towards the coil. The other armature is of the plunger type mounted for reciprocal movement inwardly and outwardly of the center of the coil, having a slightly dominant bias outwardly against the clapper armature and away from the coil face. The two armatures are thus counter balanced, with a slightly dominant bias in the direction toward the latched position. When the coil is energized, its electromagnetic force attracting the dual armatures toward the tripped position is aided by the subordinate mechanical bias towards such position. The counterbalanced dual armature arrangement of this invention makes it possible to use a coil which is small in size and yet which is constant duty rated, that is one which has high enough resistance to keep temperature rise low enough under continuous energizing voltage conditions to avoid burning out.